Five basic requirements have been defined for the COCONUT ultra-dense WDM PON architecture: (1) untouched optical distribution network, (2) no active components, (3) no intervention in the outside plan, (4) smooth upgrade from legacy systems, and (5) use of the same technology for point-to-point and point-to-multi-point links. These requirements are analysed with respect to five application scenarios: main remote, small cell backhaul/residential, business, and macro cell backhaul. The low-cost coherent techniques targeted in the project will allow for increased power budget, thus splitting ratio, distance, and a narrower grid that will enable a densification of the network as foreseen with small cell deployments.